In low-voltage driving display devices, such as vacuum fluorescent display devices, field emission display devices, or the like, ZnS:Zn phosphor is generally used as a blue phosphor. The brightness of ZnS:Zn phosphor is generally high, but it has a relatively short life-time. Therefore, oxide compounds, which have relatively low initial brightness but long life-times and good brightness-maintaining properties, have been recently developed for use as a blue phosphor. Among the oxide compounds, zinc gallate phosphor is known to have high brightness, and it is known to radiate a stable blue ray. Zinc gallate phosphor is also capable of radiating visible ray without an activator.
FIG. 2 shows the method of preparing the conventional zinc gallate phosphor. As shown in FIG. 2, ZnO, Ga.sub.2 O.sub.3, and Li.sub.3 PO.sub.4 as a flux are mixed and fired for 3 hours at 1200.degree. C. under air atmosphere. After the firing step, the fired matrix is milled, preferably by ball-milling to uniformly disperse the matrix. Then to remove excess Li.sub.3 PO.sub.4, the dispersed matrix is washed with nitric acid. The washed matrix is subjected to secondary firing for 1-3 hours at 1100.degree. C. under a reduced atmosphere and sieved to produce a low-voltage excited blue phosphor. However. the brightness of this zinc gallate phosphor is low, and a phosphor of high brightness is still required in the industry.